The present invention relates generally to ramjet propulsion devices, more particularly to ramjets having secondary fluid induction systems, such as ejectors.
A "ramjet" is a jet engine in which a fuel is combusted, having in its forward end a continuous inlet of air that depends on the speed of flight rather than on a mechanical compressor, for the compressing effect produced on the air. In ramjet powered missiles and other high performance systems, the need is to accelerate the vehicle from 0 or low velocity to a velocity where a ramjet will begin to operate on its own (Mach number about 2-3). The problem with the conventional ramjet is its inability to produce thrust at low speed. One solution is to use a turbojet or other conventional jet to get the vehicle moving initially; however, once the vehicle is moving at a velocity sufficient for the ramjet to operate, the turbojet is simply dead weight for the rest of the mission (unless it is discarded). Therefore, most ramjet-powered missiles use a rocket to accelerate to the initial velocity, although the use of rockets is very inefficient. Because it is small and compact, the rocket is attractive for use because it can be discarded. This is acceptable for some missions; however, for missions where the vehicle has other constraints such as internal stowage on the launcher the size and weight of the rocket motor becomes disadvantageous. If the rocket is integrated into the system and made to operate more like a jet engine (i.e., made so that it works with a secondary fluid (i.e., air)) the propulsion efficiency of the entire system can increase. The air-breathing action in low velocity flight is typically considered as augmentation to the rocket thrust, and is referred to as an ejector rocket or air-augmented rocket.
An "ejector" is a jet pump for withdrawing a secondary fluid from a space by movement of a primary fluid. When an ejector is combined with a ramjet as shown in FIG. 1, an "ejector ramjet" 10 is produced. Shown are a rocket 12, ejector 14, diffuser 16, combustor 18, and nozzle 19. The rocket 12 produces a primary exhaust jet 20, inducing secondary fluid 22 (typically air) into the ejector ramjet engine 10. A frictional shear boundary is shown at 24 between primary and secondary fluids.
The ejector ramjet has an advantage over the conventional ramjet in that the ejector ramjet can produce thrust at zero speed while the latter cannot. One disadvantage of the ejector ramjet is that its efficiency is not significantly better than the rocket-driven primary alone. Therefore, it would be advantageous to design a more efficient ejector ramjet to take advantage of its generation of thrust at zero speed characteristic while increasing its efficiency.
Rockets and jets follow momentum rules: EQU F=thrust=-M.sub.e V.sub.e -M.sub.i V.sub.i +(P.sub.e -P.sub.a)A.sub.e
where
A=area (ft.sup.2) PA1 M=mass flow rate (slugs/sec) PA1 P=pressure (lb/ft.sup.2) PA1 V=velocity (ft/sec) PA1 a=ambient PA1 e=exhaust PA1 i=intake PA1 .sup.n c=combined internal efficiency including combustion and thermodynamic efficiency
In static jets, the last two terms can be neglected so that F=M.sub.e V.sub.e. The input energy (E) expended to achieve the thrust is then EQU E=1/2M.sub.e V.sub.e.sup.2 .times.1/.sup.n.sub.c
where
If .sup.n.sub.c is assumed constant and the input energy is constant, i.e., primary flow is constant, F is maximized by increasing M.sub.e : EQU F=M.sub.e V.sub.e EQU F=(.sup.n.sub.c M.sub.e E).sup.1/2 =K(M.sub.e).sup.1/2
On the face of this system it would seem that thrust would be increased since M.sub.e is increased, thus increasing the propulsive efficiency of the system. The problem is that the kinetic energy of the rocket exhaust is not being equally shared among all of the fluids since the method of inducing increased air flow is frictional. Viscous shear is very inefficient for transferring energy. The rocket placed in the duct in this fashion has a very high exhaust velocity (roughly 10,000 ft/sec or more) but the air flow that is induced or pumped in can only be accelerated to about 1,000 ft/sec at 0 or relatively low vehicle speeds. This difference in velocity between the primary jet and secondary fluid does transfer momentum by shear force, but the work done on the secondary fluid (F.V.sub.s) is much less than the work removed from the primary fluid (F.V.sub.p). The balance of the energy is lost as heat. Since momentum is conserved, a situation occurs wherein momentum is transferred but little thrust augmentation is achieved, as energy transfer efficiency is only about 10%. Consequently, the assumption made above, that .sup.n.sub.c may be assumed constant, does not hold true at static conditions.
It would be advantageous if energy could be transferred more efficiently in thrust augmentation systems utilizing rockets which entrain secondary fluids, especially at 0 or very low vehicle velocities. Foa, in U.S. Pat. No. 3,046,732, compares methods of transferring mechanical energy from one flowing fluid to another flowing fluid, discussing both direct and indirect methods. Direct transfer of energy is exemplified by the highly inefficient ejector system discussed above, while indirect systems include turbomachinery, for example, the turbojet. The third type described is the direct nonsteady-flow transfer of energy by pressure waves. Berner, in U.S. Pat. No. 3,357,191 describes a combination of the "pseudo-blades" produced by the Foa system and the wave tube system, where a plurality of wave tubes downstream of the pseudo-blades is provided. Other possible relevant patents include Wygnanski et al., U.S. Pat. No. 4,257,224, who describe a mixing device which is driven to induce oscillations of two fluids about an axis normal to the mixing region flow axis, and Mueller, U.S. Pat. No. 3,925,982, who describes a fluid-dynamic shock ring for controlled flow separation in a rocket engine exhaust nozzle.